The invention relates to a motor including a cuboid piezoelectric element with a plane of vibration (xy), abounding surface extending perpendicularly to an actuating direction (x) and carrying an actuating member (2) for transmitting a force in the actuating direction (x), and an opposed bounding surface carrying a preloading element.
Such a motor is known from, for example, EP-A 755, to which U.S. Pat. No. 5,714,833 corresponds. Such a motor is used in, for example, CD drives for driving a shaft on which an arm carrying a read/write head is mounted, which arm should be moved over the CD in a radial direction. The piezoelectric element, which takes the form of a rectangular plate, has an actuating member by means of which a force can be transmitted to the shaft in an actuating direction, as a result of which the shaft is driven. The piezoelectric element is constructed in such a manner and is energized in such a way that it vibrates in a plane of vibration in which also the actuating direction lies and that the actuating member moves in accordance with an elliptical curve.
In the known motor the piezoelectric element is mounted by means of four restraining elements, two rigid restraining elements acting upon a first bounding surface of the piezoelectric element, which surface extends perpendicularly to the plane of vibration and parallel to the actuating direction, and two resilient restraining elements acting on the opposite bounding surface. The resilient restraining elements provide a preload between the piezoelectric element and a motor housing in order to ensure that the piezoelectric element, which is comparatively thin in a direction perpendicular to the plane of vibration, cannot perform a rotation about an axis perpendicular or parallel to the actuating direction and thereby change its position. However, the resilient restraining elements also give rise to a frictional force between all the restraining elements and the piezoelectric element, which force reduces the quality factor and hence the power of the motor.
It is an object of the present invention to improve the known motor with respect to the afore-mentioned drawbacks and, particularly, to increase the efficiency.
This object is achieved by restraining the piezoelectric element in directions (y,x) perpendicular to the actuating direction (x) by means of restraining elements, without preloading. It was recognized that the vibration quality of the piezoelectric element and, consequently, the power of such a motor can be increased in that, although the piezoelectric element is restrained, this restraint can be achieved with maximal slidability and without a preloading force being applied to the piezoelectric element in the directions perpendicular to the actuating direction. In the case of restraining with maximal slidability no frictional forces that could affect the vibration quality can occur between the piezoelectric element and the restraining elements. Thus, in accordance with the invention, restraining elements can be disposed in the plane of vibration, also outside the plane of vibration (for example in a plane perpendicular to the plane of vibration or in a plane which is inclined with respect to the plane of vibration), or also in a plurality of such planes. Essential is the property of the restraining elements that restraining is effected with a maximal slidability and without a preloading force being applied in a direction perpendicular to the actuating direction.
According to a preferred embodiment, the restraining elements used in the known motor have been dispensed with completely. Instead, restraining elements are arranged on the bounding surfaces of the piezoelectric element which extend parallel to the plane of vibration, which restraining elements specifically prevent the piezoelectric element from being rotated about an axis which extends perpendicularly or parallel to the actuating direction in the plane of vibration.
Further variants of the motor enable the motor to be manufactured in a simpler and cheaper manner by the use of fewer and simpler-to-manufacture parts. Mounting the electrical connections on the control electrodes of the piezoelectric element can then also be simplified.
Such a motor can be used for the purpose of moving a movable element in a direction perpendicular to the actuating direction or to drive an element which is rotatable about an axis of rotation which is oriented perpendicularly to the plane of vibration. Possible uses are for example in drive mechanisms for the read and/or write unit, particularly in optical drives such as CD or DVD drives, in which the optical unit should be moved over the optical data carrier in a radial direction with a maximal speed and with a minimal amount of space and power being required for this. Other possible uses are all situations where magnetic fields produced by conventional electric motors may affect the correct operation of an apparatus, as for example in medical uses (for example in nuclear spin tomography), or where such magnetic fields are undesirable for other reasons, as for example in shavers.